1. Field of the Invention
This invention relates to a process for the production of composite membranes which consist of a porous supporting membrane and a thin separating membrane.
2. Prior Art
Selectively permeable membranes made from different polymers are used in hyperfiltration, i.e., reverse osmosis, for the desalination of sea and brackish water, for sewage and effluent treatment and for the recovery of valuable substances from sewage and effluent. The profitability of such method depends on the efficiency of the membrane, the water flow being determined by the operating pressure and the thickness of the membrane. Efforts have been made to produce membranes as thin as possible (between 0.1 and 1 .mu.m), since the water flow is more profuse when the membrane is thin.
Either asymmetric membranes or composite membranes are used as membranes with extremely thin separating layers of less than 1 .mu.m. Asymmetric membranes, which are produced by precipitation reactions, have a dense, thin separating layer, which represents the actual membrane, and a porous substructure. The separating layer and the substructure are usually made from the same material. Conventional composite membranes consist of a porous supporting membrane, to which the actual separating membrane is applied. The supporting membrane and the separating membrane may be made of different materials. Composite membranes are either produced by interface polymerization of the components forming the separating membrane on the supporting membrane, or by applying a polymer solution to the supporting membrane followed by evaporation of the solvent.
In interface polymerization the supporting membrane is successively brought into contact with solutions of the reaction components which form the separating membrane. Since the solvents are immiscible with each other, a thin polymer film which prevents the further reaction of the two components is formed by the reaction at the interface of the solvents. However, such process can only be used for a few types of polymer synthesis and the solvents required often attack the material of the supporting membrane, destroying its structure.
The process in which the separating membrane is formed by applying a polymer solution on the supporting membrane and evaporating the solvent, requires the use of a highly diluted solvent (i.e., large amounts of solvent). After evaporation of the solvent, a homogeneous continuous polymer layer is obtained, whose thickness depends upon the polymer concentration. In such process it is important that the casting solution does not penetrate the pores of the supporting membrane and clog them. To prevent the pores from becoming clogged and to prevent the supporting membrane from being corroded, a barrier layer (which is made of a material that can later be dissolved) is applied to the porous supporting membrane and finally coated with the casting solution. If the supporting membrane is not masked in this way, only those solvents which do not effect the material of the supporting membrane may be used. The high-performance materials for the production of separating membranes are, however, only soluble in those solvents which destroy the material of the supporting membranes. Such solvents which are currently available are, for example, polysulfone, cellulose acetate and others. The casting solution can also penetrate into the pores of the supporting membrane and thus prevent the formation of a thin, dense separating membrane.
Although composite membrane production with prior masking of the supporting membrane makes it possible to produce thin, uniform separating membrane layers, total removal of the masking layer often creates major difficulties. When the composite membrane produced in such manner is used for the first time the masking material contaminates the permeate (e.g., see French Patent No. 2,322,637).
According to a conventional process, asymmetric ultrafiltration membranes are produced on a molded part of silicone rubber (see German Patent Application No. 2,132,323). A composite of the separating membrane with the support is not produced. The molded part is removed mechanically.
Another process is known in which a polymer solution is applied to a carrier and then one component of the carrier material is extracted so that a porous supporting structure occurs (see British Patent No. 1,267,167). The carrier may only contain material not contained in the separating layer.
Providing the surface of the membrane with a water-soluble film subsequent to the formation of the membrane has already been suggested as a means of protecting the sensitive surface of the membrane (see French Patent No. 2,324,338). But such process can only be used with certain polymers, and, in the production of the separating layer, the soluble polymer film does not act as a carrier.